Container and system for handling damaged nuclear fuel, and method of making the same

ABSTRACT

A container and system for handling damaged nuclear fuel, and a method of making the same. In one embodiment, the invention is a damaged fuel container having a specially designed top cap that can be detachably coupled to the elongated tubular wall by simply translating the top cap into proper position within the elongated tubular wall, wherein biased locking elements automatically lock the top cap to the elongated tubular wall. In another embodiment, the vent screens of the damaged fuel container are integrally formed rather than being separate components. In still other embodiments, the lower vent screens are arranged on an upstanding portion of the damaged fuel container. In an even further embodiment, the elongated tubular wall is formed by an extrusion process.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/239,752 filed Mar. 21, 2014, which is a U.S. national stageapplication under 35 U.S.C. §371 of PCT Application No. PCT/US12/51634,filed on Aug. 20, 2012, which claims the benefit of U.S. ProvisionalPatent Application No. 61/525,583, filed Aug. 19, 2011, the entiretiesof which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to containers and systems forhandling nuclear fuel, and specifically to containers and systems forhandling nuclear fuel whose physical integrity has been compromised, andmethods of making the same.

BACKGROUND OF THE INVENTION

Damaged nuclear fuel is nuclear fuel that is in some way physicallyimpaired. Such physical impairment can range from minor cracks in thecladding to substantial degradation on various levels. When nuclear fuelis damaged, its uranium pellets are no longer fully contained in thetubular cladding that confines the pellets from the externalenvironment. Moreover, damaged nuclear fuel can be distorted from itsoriginal shape. As such, special precautions must be taken when handlingdamaged nuclear fuel (as compared to handling intact nuclear fuel) toensure that radioactive particulate matter is contained. Please refer toUSNRC's Interim Staff Guidance #2 for a complete definition of fuel thatcannot be classified as “intact” and, thus, falls into the category ofdamaged nuclear fuel for purposes of this application. As used herein,damaged nuclear fuel also includes nuclear fuel debris.

Containers and systems for handling damaged nuclear fuel are known.Examples of such containers and systems are disclosed in U.S. Pat. No.5,550,882, issued Aug. 27, 1996 to Lehnart et al., and U.S. PatentApplication Publication No. 2004/0141579, published Jul. 22, 2004 toMethling et al. While the general structure of a container and systemfor handling damaged nuclear fuel is disclosed in each of theaforementioned references, the containers and systems disclosed thereinare less than optimal for a number of reasons, including inferiorventing capabilities of the damaged nuclear fuel cavity, difficulty ofhandling, inability to be meet tight tolerances dictated by existingfuel basket structures, lack of adequate neutron shielding, and/ormanufacturing complexity or inferiority.

Thus, a need exists for an improved container and system for handlingdamaged nuclear fuel, and methods of making the same.

SUMMARY OF THE INVENTION

In one embodiment, the invention can be a method of forming an elongatedtubular container for receiving damaged nuclear fuel, the methodcomprising: a) extruding, from a material comprising a metal and aneutron absorber, an elongated tubular wall having a container cavity;b) forming, from a material comprising a metal that is metallurgicallycompatible with the metal of the elongated tubular wall, a bottom capcomprising a first screen having a plurality of openings; and c)autogenously welding the bottom cap to a bottom end of the elongatedtubular wall, the plurality of openings of the first screen forming ventpassageways to a bottom of the container cavity.

In another embodiment, the invention can be a container for receivingdamaged nuclear fuel, the method comprising: an extruded tubular wallforming a container cavity about a container axis, the extruded tubularwall formed of a metal matrix composite having neutron absorbingparticulate reinforcement; a bottom cap coupled to a bottom end of theextruded tubular wall; a top cap detachably coupled to a top end of theextruded tubular wall; a first screen comprising a plurality of openingsthat define lower vent passageways into a bottom of the containercavity; and a second screen comprising a plurality of openings thatdefine upper vent passageways into a top of the container cavity.

In yet another embodiment, the invention can be a system for storingand/or transporting nuclear fuel comprising: a vessel comprisingdefining a vessel cavity and extending along a vessel axis; a fuelbasket positioned within the vessel cavity, the fuel basket comprising agrid forming a plurality of elongated cells, each of the cells extendingalong a cell axis that is substantially parallel to the vessel axis; andat least one elongated tubular container comprising a container cavitycontaining damaged nuclear fuel positioned within one of the cells, theelongated tubular container comprising: an extruded tubular wall forminga container cavity about a container axis, the extruded tubular wallformed of a metal matrix composite having neutron absorbing particulatereinforcement; a bottom cap coupled to a bottom end of the extrudedtubular wall; a top cap detachably coupled to a top end of the extrudedtubular wall; a first screen comprising a plurality of openings thatdefine lower vent passageways into a bottom of the container cavity; anda second screen comprising a plurality of openings that define uppervent passageways into a top of the container cavity.

In still another embodiment, the invention can be a system for storingand/or transporting nuclear fuel comprising: a vessel defining a vesselcavity and extending along a vessel axis; a fuel basket positionedwithin the vessel cavity, the fuel basket comprising a plurality ofelongated cells; an elongated tubular container positioned within one ofthe cells, the elongated tubular container comprising: an elongatedtubular wall forming a container cavity about a container axis, thetubular wall comprising a top portion having a plurality of lockingapertures and a top edge defining a top opening into the containercavity; a bottom cap coupled to a bottom end of the elongated tubularwall; a top cap comprising a plurality of locking elements that arealterable between a retracted state and an extended state, the lockingelements biased into the extended state; a first screen comprising aplurality of openings that define lower vent passageways between thevessel cavity and a bottom of the container cavity; a second screencomprising a plurality of openings that define upper vent passagewaysbetween the vessel cavity and a top of the container cavity; and the topcap and the elongated tubular wall configured so that upon the top capbeing inserted through the top opening, contact between the lockingelement and the elongated tubular wall forces the locking elements intoa retracted state, and wherein upon the locking element becoming alignedwith the locking apertures, the locking elements automatically returningthe extended state such that the locking member protrude into thelocking apertures, thereby detachably coupling the top cap to elongatedtubular wall.

In a further embodiment, the invention can be a system for storingand/or transporting nuclear fuel comprising: a vessel defining a vesselcavity and extending along a vessel axis; a fuel basket positionedwithin the vessel cavity, the fuel basket comprising a plurality ofelongated cells; an elongated tubular container comprising a containercavity for containing damaged nuclear fuel positioned within one of thecells, the elongated tubular container comprising: a first screencomprising a plurality of openings that define lower vent passagewaysbetween the vessel cavity and a bottom of the container cavity, theplurality of openings of the first screen comprising a lowermost openingthat is a first distance from a floor of the vessel cavity and anuppermost opening that is a second distance from the floor of the vesselcavity, the second distance being greater than the first distance; and asecond screen comprising a plurality of openings that define upper ventpassageways between the vessel cavity and a top of the container cavity.

In an even further embodiment, the invention can be a system for storingand/or transporting nuclear fuel comprising: a vessel defining a vesselcavity and extending along a vessel axis; a fuel basket positionedwithin the vessel cavity, the fuel basket comprising a plurality ofelongated cells; an elongated tubular container comprising a containercavity for containing damaged nuclear fuel positioned within one of thecells, the elongated tubular container comprising: a first screencomprising a plurality of openings that define lower vent passagewaysbetween the vessel cavity and a bottom of the container cavity, thefirst screen located on an upstanding portion of the elongated tubularcontainer that is substantially non-perpendicular to the vessel axis;and a second screen comprising a plurality of openings that define uppervent passageways between the vessel cavity and a top of the containercavity.

In a still further embodiment, the invention can be a damaged fuelcontainer, or system incorporating the same, in which the one or more ofthe screens of the container are integrally formed into the body of thecontainer.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is an isometric view of a damaged fuel container according to anembodiment of the present invention;

FIG. 2 a bottom perspective view of a bottom portion of the damaged fuelcontainer of FIG. 1;

FIG. 3 is a top perspective view of a top portion of the damaged fuelcontainer of FIG. 1;

FIG. 4 is a longitudinal cross-sectional schematic of the damaged fuelcontainer of FIG. 1 taken along the container axis, wherein a middleportion of the damaged fuel container has been omitted;

FIG. 5 is a close-up longitudinal cross-sectional schematic of thebottom portion of the damaged fuel container of FIG. 1;

FIG. 6 is an isometric view of the top cap of the damaged fuel containerof FIG. 1, wherein the top cap has been removed;

FIG. 7 is a longitudinal cross-sectional schematic of the top cap ofFIG. 5 positioned above the elongated tubular wall of the damaged fuelcontainer for detachable coupling thereto;

FIG. 8 is a longitudinal cross-sectional schematic wherein the top capof FIG. 5 has been partially inserted through a top opening of theelongated tubular wall of the damaged fuel container;

FIG. 9 is a longitudinal cross-sectional schematic wherein the top capof FIG. 5 has been slidably inserted into the container cavity of theelongated tubular wall, and wherein the locking elements of the top caphave been forced into a fully retracted state due to contact with theelongated tubular wall;

FIG. 10 is a top view of a system according to an embodiment of thepresent invention, wherein a loaded damaged fuel container of FIG. 1 andintact fuel assemblies are schematically illustrated therein;

FIG. 11 is cross-sectional view taken along view XI-XI of FIG. 10; and

FIG. 12 is a close-up view of area XII-XII of FIG. 11.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description of the illustrated embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, any reference to direction or orientation is merelyintended for convenience of description and is not intended in any wayto limit the scope of the present invention. Relative terms such as“lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,”“down,” “top” and “bottom” as well as derivatives thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) should be construed torefer to the orientation as then described or as shown in the drawingunder discussion. These relative terms are for convenience ofdescription only and do not require that the apparatus be constructed oroperated in a particular orientation unless explicitly indicated assuch. Terms such as “attached,” “affixed,” “connected,” “coupled,”“interconnected,” and similar refer to a relationship wherein structuresare secured or attached to one another either directly or indirectlythrough intervening structures, as well as both movable or rigidattachments or relationships, unless expressly described otherwise.Moreover, the features and benefits of the invention are illustrated byreference to the exemplified embodiments. Accordingly, the inventionexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features; the scope of theinvention being defined by the claims appended hereto.

Referring first to FIGS. 1-4 concurrently, a damaged fuel container(“DFC”) 100 according to an embodiment of the present invention isillustrated. The DFC 100 incorporates an inventive design (and is formedby an inventive method) that allows high density packaging of damagedfuel in pressure vessels, such as metal casks or multi-purpose canisters(described in greater detail below). The DFC 100 can be used to packagedamaged nuclear fuel at nuclear reactors, such as the Fukushima Daiichisite. The DFC 100 can be used to safely containerize nuclear fuel ofcompromised cladding integrity and is a unitary waste package for thefuel that may be in various stages of dismemberment ranging from minorcracks in the cladding to its substantial degradation. As described ingreater detail below, the DFC 100 is designed to be loaded with damagednuclear fuel and positioned within a fuel basket which, in turn, ishoused in a pressure vessel such as a metal cask or a multi-purposecanister.

The DFC 100 is an elongated tubular container that extends along acontainer axis C-C. As will become more apparent from the descriptionbelow, the DFC 100 is specifically designed so as to not form afluid-tight container cavity 101 therein. This allows the containercavity 101 of the DFC 100, and its damaged nuclear fuel payload, to beadequately dried for dry storage using standard dry storage dehydrationprocedures. Suitable dry storage dehydration operations and equipmentthat can be used to dry the DFC 100 (and the system 1000) are disclosedin, for example: U.S. Patent Application Publication No. 2006/0288607,published Dec. 28, 2006 to Singh; U.S. Patent Application PublicationNo. 2009/0158614, published Jun. 2, 2009 to Singh et al.; and U.S.Patent Application Publication No. 2010/0212182, published Aug. 22, 2010to Singh. While a fluid-tight boundary is not formed by the DFC 100, theDFC 100 (when fully assembled as shown in FIGS. 1-4) creates aparticulate confinement boundary for its damaged nuclear fuel payload,thereby preventing radioactive particles and debris from escaping thecontainer cavity 101.

The DFC 100 generally comprises an elongated tubular wall 10, a bottomcap 20 and a top cap 30. In one embodiment, the elongated tubular wall10 is formed of a material comprising a metal and a neutron absorber. Asused herein the term “metal” includes metals and metal alloys. Incertain embodiments, suitable metals may include without limitationaluminum, steel, lead, and titanium while suitable neutron absorbers mayinclude without limitation boron, boron carbide and carborundem. As usedherein, the term “aluminum” includes aluminum alloys. In one specificembodiment, the metal is an aluminum and the neutron absorber materialis boron or boron carbide. In other embodiments, the elongated tubularwall 10 is formed entirely of a metal matrix composite having neutronabsorbing particulate reinforcement. Suitable metal matrix compositeshaving neutron absorbing particulate reinforcement include, withoutlimitation, a boron carbide aluminum matrix composite material, a boronaluminum matrix composite material, a boron carbide steel matrixcomposite material, a carborundum aluminum matrix composite material, acarborundum titanium matrix composite material and a carborundum steelmatrix composite material. Suitable aluminum boron carbide metal matrixcomposites are sold under the name Metamic® and Boralyn®. The use of analuminum-based metal matrix composite ensures that the DFC 100 will havegood heat rejection capabilities.

The boron carbide aluminum matrix composite material of which theelongated tubular wall 10 is constructed, in one embodiment, comprises asufficient amount of boron carbide so that the elongated tubular wall 10can effectively absorb neutron radiation emitted from the damage nuclearfuel loaded within the container cavity 101, thereby shielding adjacentnuclear fuel (damaged or intact) in the fuel basket 400 from one another(FIG. 10). In one embodiment, the elongated tubular wall 10 isconstructed of an aluminum boron carbide metal matrix composite materialthat is greater than 25% by volume boron carbide. In other embodiments,the elongated tubular wall 10 is constructed of an aluminum boroncarbide metal matrix composite material that is between 20% to 40% byvolume boron carbide, and more preferably between 30% to 35%. Of course,the invention is not so limited and other percentages may be used. Theexact percentage of neutron absorbing particulate reinforcement requiredto be in the metal matrix composite material will depend on a number offactors, including the thickness of the elongated tubular wall 10, thespacing/pitch between adjacent cells within the fuel basket 400 (FIG.10), and the radiation levels of the damaged nuclear fuel. As will bediscussed in greater detail below, the elongated tubular wall 10 isformed by an extrusion process in certain embodiments and, thus, the DFC100 can be considered an extruded tubular container in such embodiments.Extrusion is preferred because it results in an elongated tubular wall10 that is free of bending or warping that can be caused by weldingprocesses that are used to create tubes.

The elongated tubular wall 10 extends along the container axis C-C froma top end 11 to a bottom end 12. The top end 11 terminates in a top edge13 while the bottom end 12 terminates in a bottom edge 14. The elongatedtubular wall 10 also comprises an outer surface 15 and an inner surface16 that forms a container cavity 101. The top edge 13 defines a topopening 17 that leads into the container cavity 101.

The elongated tubular wall 10 comprises a top portion 18 and a bottomportion 19. In the exemplified embodiment, the bottom portion 19 extendsfrom the bottom edge 14 to a transition shoulder 21 while the topportion 18 extends from the transition shoulder 21 to the top edge 13.The top portion 19, in the exemplified embodiment, is an upper sectionof the elongated tubular wall 10 that flares slightly outward movingfrom the transition shoulder 21 to the top edge 13. Thought of anotherway, the top portion 19 of the elongated tubular wall 10 has atransverse cross-section that gradually increases in size moving fromthe transition shoulder 21 to the top edge 13. The bottom portion 18, inthe exemplified embodiment, has a substantially constant transversecross-section along its length, namely from the bottom edge 14 to thetransition shoulder 21. In other embodiments, the top portion 19 canalso have a transverse cross-section that is substantially constantalong its length from the transition shoulder 21 to the top edge 13. Insuch an embodiment, the transverse cross-section of the top portion canbe larger than the transverse cross-section of the bottom portion 18. Instill other embodiments, the elongated tubular wall 10 may have asubstantially constant transverse cross-section along its entire lengthfrom the bottom edge 14 to the top edge 13. In such an embodiment, theelongated tubular wall 10 will be devoid of a transition shoulder 21 andthe top and bottom portions 18, 19 would have no physical distinction.

In the exemplified embodiment, the elongated tubular wall 10 has asubstantially constant thickness along its entire length. In oneembodiment, the elongated tubular wall 10 has a wall thickness between 1mm to 3 mm, with about 2 mm being preferred. Of course, the invention isnot so limited and the elongated tubular wall 10 can have wall thicknessthat is variable and of different empirical values and ranges.

The inner surface 16 of the elongated tubular wall 10 defines thecontainer cavity 101. In the exemplified embodiment, the portion of thecontainer cavity 101 defined by the bottom portion 18 has a transversecross-section that is substantially constant in size while the portionof the container cavity 101 defined by the top portion 19 has atransverse cross-section that increases in size moving from thetransition shoulder 21 to the top edge 13.

In the exemplified embodiment, the elongated tubular wall 10 has atransverse cross-section that is substantially rectangular in shapealong its entire length from the bottom edge 14 to the top edge 13.Similarly, the container cavity 101 also has a transverse cross-sectionthat is substantially rectangular in shape along its entire length. Ofcourse, the transverse cross-sections can be other shapes in otherembodiments, and can even be dissimilar shapes between the top andbottom portions 18, 19.

The bottom cap 20 is fixedly coupled to the bottom end 12 of theelongated tubular wall 10 while the top cap 30 is detachably coupled tothe top end 11 of the elongated tubular wall 10. More specifically, thebottom cap 20 is coupled to the bottom edge 14 of the elongated tubularwall 10. As will be described in greater detail below, in theexemplified embodiment, the bottom cap 20 is fixedly coupled to thebottom end 12 of the elongated tubular wall 10 by an autogenous weldingtechnique, such as by friction stir welding. In other embodiments, thebottom cap 20 is fixedly coupled to the bottom end 12 of the elongatedtubular wall 10 using other connection techniques.

The bottom cap 20, in certain embodiments, is formed of a materialcomprising a metal that is metallurgically compatible with the metal ofthe elongated tubular wall 10 for welding. In one embodiment, the bottomcap is formed of aluminum. The bottom cap 20, in a preferred embodiment,is formed by a casting process.

The bottom cap 20 comprises a plurality of first screens 22. Each of thefirst screens 22 comprises a plurality of openings 23 that define lowervent passageways into a bottom 102 of the container cavity 101. While inthe exemplified embodiment the first screens 22 are incorporated intothe bottom cap 20, the first screens 22 can be incorporated into thebottom end 12 of the elongated tubular wall 10 in other embodiments.Furthermore, while the exemplified DFC 100 comprises four first screensin the exemplified embodiment, more or less first screens 22 can beincluded in other embodiments.

In one embodiment, the openings 23 of the first screens 22 are smallenough so that radioactive particulate matter cannot pass therethroughbut are provided in sufficient density (number of openings/area) toallow sufficient venting of air, gas or other fluids through thecontainer cavity 101. In one embodiment, the openings 23 have a diameterin a range of 0.03 mm to 0.1 mm, and more preferably a diameter of about0.04 mm. The openings 23 may be provided for each of the first screens22, in certain embodiments, to have a density of 200 to 300 holes persquare inch. The invention, however, is not limited to any specificdimensions or hole density unless specifically claimed.

In the exemplified embodiment, the first screens 22 are integrallyformed into a body 24 of the bottom cap 20 by creating the openings 23directly into the body 24 of the bottom cap 20. The openings 23 can beformed into the body 24 of the bottom cap 20 by punching, drilling orlaser cutting techniques. In one embodiment, it is preferred to form theopenings using a laser cutting technique. Laser cutting allows very fineopenings 23 to be formed with precision and efficiency. In alternateembodiments, the openings of the first screens 22 may not be integrallyformed into the bottom cap 20 (or the elongated tubular wall 10).Rather, larger through holes can be formed in the bottom cap 20 that arethen covered by separate first screens 22, such as wire mesh screens.

Referring now to FIGS. 2 and 5 concurrently, the bottom cap 20 generallycomprises a floor plate 25 and an oblique wall 26 extending upward froma perimeter of the floor plate 25. In the exemplified embodiment, theoblique wall 26 is integrally formed with the floor plate 25, forexample, during the casting formation process. The oblique wall 26 is arectangular annular wall that forms a tapered end of the DFC 100, whichhelps with inserting the DFC 100 into a cell 403B of the fuel basket 400(FIGS. 10 and 11). The oblique wall 26 extends oblique to the containeraxis C-C and terminates in an upper edge 27. The upper edge 27 of theoblique wall 26 is coupled to the bottom edge 14 of the elongatedtubular wall 10 by an autogenous butt weld 29 that seals the interfaceand integrally couples the components together so as to produce ajunction that is smooth with the outer surface 15.

The floor plate 25 comprises a top surface 28 that forms a floor of thecontainer cavity 101. As can be seen in FIG. 5, one of the first screens22 is located on each of the four sections of the oblique wall 26, whichcollectively form its rectangular transverse cross-sectional shape. Theoblique wall 26 is an upstanding portion of the DFC 100. By locating thefirst screens 22 on an upstanding portion of the DFC 100 (rather than aportion that only has a horizontal component, such as the floor plate25), the openings 23 of the first screens 23 are less susceptible tobecoming clogged from particulate matter from the damaged nuclear fuel.Moreover, the openings 23 do not become choked-off (i.e., blocked) whenthe DFC 100 is supported upright in a fuel basket 400 and the floorplate 25 is in surface contact with a floor 505 of the vessel 500 (FIG.12). In certain embodiments, an additional first screen 22 may be addedto the floor plate 25 of the bottom cap 20 to ensure adequate leakage ofretained water.

The openings 23 of each of the first screens 22 comprise a lowermostopening(s) 23A and an uppermost opening(s) 23C. The lowermost opening23A is located a first axial distance d₁ above the floor 28 of thecontainer cavity 101 while the uppermost most opening 23C is located asecond distance d₂ above the floor 28 of the container cavity 101. Thesecond distance d₂ is greater than the first distance d₁. As discussedbelow, the DFC 100, in certain embodiments, is intended to be orientedso that the container axis C-C is substantially vertical when the DFC100 is positioned within the fuel basket 400 of the vessel 500 fortransport and/or storage. Thus, in the exemplified embodiment, both thelowermost and uppermost openings 23A, C are located a vertical distanceabove the floor 28 of the container cavity 101. As a result, the firstscreens 22 are unlikely to become clogged by settling particulate debrisas each of d₁ and d₂ are vertical distances.

As mentioned above, it is beneficial to have the first screens 22located on an upstanding portion of the DFC 100, which in theexemplified embodiment is the oblique wall 26 of the bottom cap 20. Inother embodiments, the bottom cap 20 is designed so that the wall 26 isnot oblique to the container axis C-C but rather substantially parallelthereto. In such and embodiment, the first screens 22 are located onthis vertical annular wall of the bottom cap 20. In still anotherembodiment, the bottom cap 20 may simply be a floor plate without anysignificant upstanding portion. In such an embodiment, the first screens22 can be located on the bottom end 12 of the elongated tubular wall 10itself, which would be considered the upstanding portion that issubstantially parallel to container axis C-C. Of course, in suchembodiments, the upstanding portion of the elongated tubular wall 10 onwhich the first screens 22 are located can be oriented oblique to thecontainer axis C-C.

Referring now to FIGS. 3-4 and 6 concurrently, the details of the topcap 30, along with its detachable coupling to the elongated tubular body10 will be discussed in greater detail. The top cap 30 is shaped toprovide a strong attachment location for lifting the loaded DFC 100. Ahandle 31 is fixedly coupled to the top cap 30 and extends upward from atop surface 32 of the top cap 30 so that the DFC 100 can be easilyhandled by a crane or other handling equipment. As can be seen, when thetop cap 30 is detachably coupled to the elongated tubular wall 10 (shownin FIGS. 3-4), the entirety of the top cap 30 is disposed within the topportion 19 of the elongated tubular wall 10. A portion of the handle 31,however, protrudes axially from the top edge 13 of the elongated tubularwall 13. Nonetheless, the entirety of the handle 31 is located fullywithin a transverse perimeter defined by the top edge 13 of theelongated tubular wall 10 (viewed from a plane that is substantiallyperpendicular to the container axis C-C). As a result, the handle 31 canbe easily grabbed by lifting mechanisms when the DFC 100 is fullyinserted into a fuel cell of a fuel rack, without the grid 401 of thefuel basket 400 interfering with the lifting mechanism (FIGS. 10 and11).

The top cap 30 comprises a body 33. In one embodiment, the body 33 isformed of any of the materials described above for the elongated tubularwall 10. In another embodiment, the body 33 is formed of any of thematerials described above for the bottom cap 20.

The top cap 30 has a bottom surface 34, a top surface 32 and aperipheral sidewall 35. The peripheral sidewall 35 comprises a chamferedportion 36 at a lower edge thereof to facilitate insertion into the topopening 17 of the elongated tubular wall 10. The top cap 30 has atransverse cross-sectional shape that is the same as the transversecross-sectional shape of the container cavity 101.

A plurality of locking elements 37 protrude from the peripheral sidewall35 of the top cap 30 and, as discussed in greater below, are alterablebetween a fully extended state (shown in FIGS. 3-4 and 6) and a fullyretracted state (shown in FIG. 9) to facilitate repetitive coupling anduncoupling of the top cap 30 to the elongated tubular wall 10. In theexemplified embodiment, the locking elements 37 are spring-loaded pins.In other embodiments, the locking elements 37 can be tabs,protuberances, clamps, tangs, and other known mechanisms for lockingcomponents together

The top cap 30 also comprises a second screen 38. The second screen 38comprises a plurality of openings 39 that define upper vent passagewaysinto a top 103 of the container cavity 101. While in the exemplifiedembodiment the second screen 38 is incorporated into the top cap 30, thesecond screen 38 can be incorporated into the elongated tubular wall 10at a position below where the top cap 30 couples to the elongatedtubular wall 10 in other embodiments.

In one embodiment, the openings 39 of the top cap are small enough sothat radioactive particulate matter cannot pass therethrough but areprovided in sufficient hole density (number of openings/area) to allowsufficient venting of air and gases (or other fluids) through thecontainer cavity 101. In one embodiment, the openings 39 have a diameterin a range of 0.03 mm to 0.1 mm, and more preferably a diameter of about0.04 mm. The openings 39 may be provided for the second screen 38, incertain embodiments, to have a density of 200 to 300 holes per squareinch. The invention, however, is not limited to any specific dimensionsor hole density of the openings 39 unless specifically claimed.

In the exemplified embodiment, the second screen 38 is integrally formedinto the body 33 of the top cap 30 by creating the openings 39 directlyinto the body 33 of the bottom cap 20. The openings 39 can be formedinto the body 33 of the top cap 30 by punching, drilling or lasercutting techniques. In one embodiment, it is preferred to form theopenings 39 using a laser cutting technique. Laser cutting allows veryfine openings 39 to be formed with precision and efficiency. Inalternate embodiments, the openings 39 of the second screen 38 may notbe integrally formed into the top cap 30 (or the elongated tubular wall10). Rather, larger through holes can be formed in the top cap 30 thatare then covered by a separate second screen(s), such as a wire meshscreen(s).

Referring now to FIGS. 7-9, additional details of the locking elements37 of the top cap 30, and the coupling of the top cap 30 to theelongated tubular wall 10, will be described. As mentioned above, thelocking elements 37 are alterable between a fully extended state (FIG.7) and a fully retracted state (FIG. 9).

Referring solely now to FIG. 7, each of the locking elements 37 isbiased into the fully extended state by a resilient element 40, which inthe exemplified embodiment is a coil spring that is fitted over a shaftportion 41 of the locking element 37. In the exemplified embodiment, thesprings 40 bias the locking elements 37 into the extended state throughcontact with a first wall 43 of the top cap 30 on one end and a flange44 of the shaft portion 41 of the locking element 37 on the other end.Overextension of the locking elements 37 out of the peripheral sidewall35 is prevented by contact interference between the flanges 44 of theshaft portions 41 and second walls 45 of the top cap. Upon theapplication of adequate force to the locking elements 37, the springforce of the springs 40 is overcome and each of the locking elements 37will translate along its locking element axis L-L (FIG. 4) to the fullyretracted state. In the exemplified embodiment, the locking element axesL-L are substantially perpendicular to the container axis C-C. Incertain embodiments, the internal chambers 45 in which the springs 40and portions of the locking elements 37 nest are hermetically sealed.This can be accomplished by incorporating a suitable gasket about theshaft portion 41 of the locking element at the peripheral sidewall 35.In the exemplified embodiment, a locking element 37 is provided on eachone of the four sections of the peripheral sidewall 35 and are centrallylocated thereon at the cardinal points.

As described in greater detail below, the locking elements 37 are forcedfrom the fully extended state to the fully retracted state due tocontact between the extruded tubular wall 10 and the locking elements 37during insertion of the top cap 30 into the container cavity 101. As canbe seen in FIG. 7, the portion of the container cavity 101 defined bythe top portion 19 of extruded tubular wall has a transversecross-section that gradually tapers (i.e. decreases in size) moving away(i.e., downward in the illustration) from a top edge 13 of the elongatedtubular wall 10. Thus, the container cavity 101 has a transversecross-section A₁ at the top opening 17 that is greater than thetransverse cross-section A₂ of the container cavity 101 at an axialposition immediately above locking apertures 50 formed into theelongated tubular wall 10.

As mentioned above, the locking elements 37 are biased into a fullyextended state and, thus, protrude from all four sections of theperipheral sidewall 35. As a result of the protruding locking elements37, the top cap 37 has an effective transverse cross-section A₃ when thelocking elements 37 are in the fully extended state. The DFC 100 isdesigned, in the exemplified embodiment, so that the effectivetransverse cross-section A₃ of the top cap 30 is the same as or smallerthan the transverse cross-section A₁ of the top opening 17 of theinternal cavity 101. The effective transverse cross-section A₃ of thetop cap 30, however, is greater than the transverse cross-section A₂ ofthe container cavity 101 at the axial position immediately above lockingapertures 50.

Referring now to FIG. 8, as a result of the relative dimensionsdescribed immediately above, when the top cap 30 is initially alignedwith and lowered into the top opening 17 of the container cavity 101,the top cap 70 (including the locking elements 70) can pass through thetop opening 17 while the locking elements 37 remain in the fullyextended state. Thought of another way, the top edge 13 defines the topopening 17 so as to have a transverse cross-section through which thetop cap 30 can be inserted while the locking elements 37 are in thefully extended state.

As the top cap 30 continues to be inserted (i.e., lowered in theillustration), the locking elements 37 come into contact with the innersurface 16 of the top portion 19 of the elongated tubular wall 10 thatdefines that portion of the container cavity 101. Due to the fact thatthe inner surface 16 is sloped such that the transverse cross-section ofthe container cavity 101 continues to decrease with distance from thetop edge 13, the locking elements 37 are further forced into retractionby the inner surface 16 of the elongated tubular wall 10 until a fullyretracted state is achieved at the axial position immediately abovelocking apertures 50 (FIG. 9).

Referring to FIG. 9, the locking elements 37 are at the axial positionimmediately above locking apertures 50 of the elongated tubular wall 10and are in the fully retracted state. In the fully retracted statestate, the springs 40 are fully compressed and the locking elements 37have been translated inward along the locking element axis L-L. Aslowering of the top cap 30 is continued, the locking elements 37 becomealigned with the locking apertures 50 of the elongated tubular wall 10and are automatically returned back into the fully extended state inwhich the locking elements 37 protrude into the locking apertures 50 dueto the bias of the springs 40 (shown in FIG. 4). As a result of thelocking elements 37 protruding into the locking apertures 50, the topcap 30 is coupled to the elongated tubular wall 10 so that the DFC 100can be lifted by the handle 31. The locking elements 37 cannot be forcedback into the retracted state due to contact with the edges that definethe locking apertures 50. In other words, once the top cap 30 is coupledto the elongated tubular wall 10 as described above, the lockingelements 37 cannot be retracted by applying a lifting or pulling force(i.e. an axial force along the container axis C-C) to the top cap 30.Thus, a secure connection between the top cap 30 and the elongatetubular wall 10 is provided. In order to remove the top cap 30 from theelongated tubular wall 10, a tool is required to unlock the top cap 30from the elongated tubular wall 10 by pressing the locking elements 37radially inward along their locking element axes L-L. In the exemplifiedembodiment, the locking apertures 50 are through-holes and, thus, thelocking elements 37 can be pressed inward by the access provided tto thelocking elements 37 by the locking apertures 50.

The exemplified embodiment is only one structural implementation inwhich the top cap 30 and the elongated tubular wall 10 are configured sothat upon the top cap 30 being inserted through the top opening 17,contact between the locking elements 37 and the elongated tubular wall10 forces the locking elements 37 into a retracted state. In otherembodiments, the effective transverse cross-section A₃ of the top cap 30may be larger than the transverse cross-section A₁ of the top opening 17of the internal cavity 101. In such an embodiment, the lower edges ofthe locking elements 37 can be appropriately chamfered and/or rounded sothat upon coming into contact with the top edge 13 of the elongatedtubular wall 10 during lowering, contact between the lower edges of thelocking elements 37 and the top edge 13 of the elongated tubular wall 10forces the locking elements 37 to translate inward along their lockingelement axes L-L. In other embodiments, the top edge 13 of the elongatedtubular wall 10 may be appropriately chamfered to achieve the desiredtranslation of the locking elements 37.

Referring now to FIGS. 10-12 concurrently, a system 1000 for storingand/or transporting damaged nuclear fuel is illustrated according to anembodiment of the present invention. The system 1000 generally comprisesa vessel 500, a fuel basket 400 and at least one of the DFCs 100described above. The vessel 500, when fully assembled, forms afluid-tight vessel cavity 501 in which the fuel basket 400, the DFC 100containing damaged nuclear fuel and intact nuclear fuel 50 are housed(in FIG. 10, the loaded DFC 100 and the intact nuclear fuel 50 areschematically illustrated for simplicity). Thus, the vessel 500 can beconsidered a pressure vessel that forms a fluidic containment boundaryabout the vessel cavity 501. In the exemplified embodiment, the vessel500 is a canister, such as a multi-purpose canister. In embodiments,where the vessel is an MPC, the system 100 may also comprises anoverpack cask, such as an above-ground or below-ground ventilatedvertical overpack. In other embodiments, the vessel 500 may be a metalcask.

The vessel 500 comprises a cylindrical shell 502, a lid plate 503 and afloor plate 504. The lid plate 503 and the floor plate 504 are sealwelded to the cylindrical shell 502 so to form the hermetically sealedvessel cavity 501. A top surface 505 of the floor plate 504 forms afloor of the vessel cavity 501. The vessel 500 extends along a vesselaxis V-V, which is arranged substantially vertical during normaloperation and handling procedures.

The fuel basket 400 is positioned within the vessel cavity 502 andcomprises a gridwork 401 forming a plurality of elongated cells 403A-B.In the exemplified embodiment, the gridwork 401 is formed by a pluralityof intersecting plates 402 that form the cells 403A-B. In oneembodiment, the plates 402 that form the gridwork 401 are formed ofstainless steel. Because the elongated tubular wall 10 of the DFC 100 ismade of a boron carbide aluminum matrix composite material, or a boronaluminum matrix composite material, and the gridwork 401 is made ofstainless steel, there is no risk of binding from the cohesion effect ofmaterials of identical genre.

Each of the elongated cells 403A-B extend along a cell axis B-B that issubstantially parallel to the vessel axis V-V. The plurality of cells403A-B comprises a first group of cells 403A that are configured toreceive intact nuclear fuel 50 and a second group of cells 403Bconfigured to receive DFCs 100 containing damage nuclear fuel. Each ofthe cells 403A of the first group comprise neutron absorbing linerpanels 404 while the each of the cells 403B of the second group are freeof the neutron absorbing liner panels 404. In one embodiment, theneutron absorbing liner panels 404 can be constructed of the samematerial that is described above for the elongated tubular wall 10.

Because the elongated tubular wall 10 of the DFC 100 incorporate neutronabsorber as described above, the cells 403B of the fuel basket 400 thatare to receive the DFCs 100 do not require such neutron absorber plates404, leading to an increased cell cavity size which is large enough toenable free insertion or extraction of the DFC 100 from the fuel basket400. In certain embodiments, the cell opening of the cells 403B is 6.24inches, which means that there is a ¼ inch lateral gap between the DFC100 and the grid that forms the storage cell 403B. Moreover, because theDFC 100 is extruded and the cells 403A-B of the fuel basket 400 are ofhoneycomb construction made of thick plate stock (¼ inch wall), there isa high level of confidence that the DFCs 100 can be inserted into thestorage cells 403B without interference. In the exemplified embodiment,all of the cells 403A-B have the same pitch therebetween.

Referring now to FIGS. 11 and 12, each of the DFCs 100 is loaded intoone of the cells 403B by aligning the DFC 100 with the cell 403B andlowering the DFC 100 therein until the floor plate 25 of the DFC 100comes into surface contact with and rests on the top surface 505 of thefloor plate 504 of the vessel 500. When positioned within the cell 403B,the container axis C-C of the DFC 100 is substantially parallel to thecell axis B-B and, in certain embodiments, substantially coaxialtherewith.

As mentioned above, the cell axis B-B is substantially parallel to thevessel axis V-V. Thus, when the DFC 100 is loaded within the cell 403B,the oblique wall 26 of the bottom cap 20 is oblique to both the cellaxis B-B and the vessel axis V-V. As mentioned above, the top surface505 of the floor plate 504 forms a floor of the vessel cavity 501. Thus,when the DFC 100 is loaded within the cell 403B, the lowermostopening(s) 23A of the first vent(s) 22 is a distance d₃ above the floor505 of the vessel 500 while the uppermost opening(s) 23C of the firstvent(s) 22 is a distance d₄ above the floor 505 of the vessel 500.

In summary, the DFC 100 of the present invention fits in the storagecell 403B with adequate clearance. The DFC 100 also provides adequateneutron absorption to meet regulatory requirements. The DFC 100 alsoconfines the particulates but allow water and gases to escape freely.The DFC 100 also features a robust means for handling and includes asmooth external surface to mitigate the risk of hang up during insertionin or removal from the storage cell 403 B. The DFC also provides minimalresistance to the transmission of heat from the contained damagednuclear fuel. The loaded DFC 100 can be handled by a grapple from theFuel Handling Bridge. All lifting appurtenances are designed to meetANSI 14.6 requirements with respect to margin of safety in loadhandling. Specifically, the maximum primary stress in any part of theDFC 100 will be less than its Yield Strength at 6 times the dead weightof the loaded DFC,W. and less than the Ultimate Strength at 10 times W.

The table below provides design data for one embodiment of the DFC 100.

DFC: Design Data Outer Dimension 152 mm (5.99″) Corner Radius 6 mm(0.24″ nominal) Wall Thickness 2.0 mm (0.079″) DFC Cell I.D. 148 mm(5.83″) Total Height 4680 mm (184.25″) Boron Carbide Concentration 32%(nominal) Empty Weight, Kg 25 (55 lbs) Permissible Planar AverageEnrichment 4.8%

A method of manufacturing the DFC 100 according to an embodiment of thepresent invention will now be described. First, the elongated tubularwall 10 is formed via an extrusion process using a metal matrixcomposite having neutron absorbing particulate reinforcement. A boroncarbide aluminum matrix composite material is preferred. At this stage,the extruded elongated tubular wall 10 (and the container cavity 101)has a substantially constant transverse cross-section, with theelongated tubular wall 10 also having a substantially uniform wallthickness. The elongated tubular wall 10 is then taken and a portionthereof is expanded so that the container cavity 101 has an increasedtransverse cross-section, thereby forming the top portion 19 and thebottom portion 18 elongated tubular wall 10. Expansion of the containercavity 101 (which can also be considered expansion of the elongatedtubular wall 10) can be accomplished using a swaging process using anappropriate mandrel, die and/or press. Said swaging process can be a hotwork in certain embodiments. In an alternate embodiment, the differencesizes in transverse cross-section of the container cavity 101 can beaccomplished by performing a drawing process to reduce the bottomportion 18 of the elongate tubular wall 10.

The locking apertures 50 are then formed into the top portion of theelongated tubular wall 10 via a punching, drilling, or laser cuttingtechnique.

The bottom cap 20 is then formed. Specifically, the bottom cap 20 isformed by casting aluminum to form the cap body 24. The plurality ofopenings 23 are then integrally formed therein using a laser cuttingprocess to form the first screens 22 on the oblique wall 26.

The bottom cap 20 is then autogenously welded to the bottom end 12 ofthe elongated tubular wall 10. More specifically, the bottom cap 20 isbutt welded to the bottom end 12 of the elongated tubular wall 10 toproduce a weld junction that is smooth with the outer surface 15 of theelongated tubular wall 10. A friction stir weld technique may be used.

The top cap 30 is then formed and coupled to the elongated tubular wall10 as described above.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques. It is tobe understood that other embodiments may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Thus, the spirit and scope of the inventionshould be construed broadly as set forth in the appended claims.

What is claimed is:
 1. A method of forming an elongated tubularcontainer for receiving damaged nuclear fuel, the method comprising: a)extruding, from a material comprising a metal and a neutron absorber, anelongated tubular wall having a container cavity; b) forming, from amaterial comprising a metal that is metallurgically compatible with themetal of the elongated tubular wall, a bottom cap comprising a firstscreen having a plurality of openings; and c) autogenously welding thebottom cap to a bottom end of the elongated tubular wall, the pluralityof openings of the first screen forming vent passageways to a bottom ofthe container cavity.
 2. The method according to claim 1 wherein step b)further comprises: b-1) casting a body of the bottom cap; and b-2)integrally forming the plurality of openings into the body of the bottomcap to form the first screen.
 3. The method according to claim 2 whereinthe plurality of openings are integrally formed by laser cutting theplurality of openings into the body of the bottom cap.
 4. The methodaccording to claim 1 wherein the elongated tubular wall is formed of ametal matrix composite having neutron absorbing particulatereinforcement.
 5. The method according to claim 4 wherein the metalmatrix composite having neutron absorbing particulate reinforcement is aboron carbide aluminum matrix composite material.
 6. The methodaccording to claim 1 wherein the bottom cap is formed of aluminum. 7.The method according to claim 1 wherein step c) further comprises: c-1)butt welding the bottom cap to the bottom end of the elongated tubularwall to produce a weld junction that is smooth with an outer surface ofthe elongated tubular wall.
 8. The method according to claim 1 whereinthe container cavity of the elongated tubular wall resulting from stepa) has a substantially constant transverse cross-section.
 9. The methodaccording to claim 8 further comprises: expanding a portion of thetransverse cross-section of the container cavity along a top portion ofthe elongated tubular wall.
 10. The method according to claim 9 whereinthe expanded portion of the transverse cross-section of the containercavity tapers moving away from a top edge of the elongated tubular wall.11. The method according to claim 1 further comprising: d) forming a topcap having a second screen comprising a plurality of openings; and e)detachably coupling the top cap to the elongated tubular wall, theplurality of openings of the first cap forming vent passageways into atop of the container cavity.
 12. The method according to claim 11wherein step e comprises: e-1) sliding the top cap into an open top endof the container cavity, wherein locking elements of the top cap thatare normally biased into an extended state are forced into a retractedstate due to contact with the elongated tubular wall during saidsliding; and e-2) upon the locking elements of the top cap becomingaligned with locking apertures of the elongated tubular container, thelocking elements automatically returning to the extended state such thatthe locking elements protrude into the locking apertures.
 13. A systemfor storing and/or transporting nuclear fuel comprising: a vesselcomprising defining a vessel cavity and extending along a vessel axis; afuel basket positioned within the vessel cavity, the fuel basketcomprising a grid forming a plurality of elongated cells, each of thecells extending along a cell axis that is substantially parallel to thevessel axis; and at least one elongated tubular container comprising acontainer cavity containing damaged nuclear fuel positioned within oneof the cells, the elongated tubular container comprising: an extrudedtubular wall forming a container cavity about a container axis, theextruded tubular wall formed of a metal matrix composite having neutronabsorbing particulate reinforcement; a bottom cap coupled to a bottomend of the extruded tubular wall; a top cap detachably coupled to a topend of the extruded tubular wall; a first screen comprising a pluralityof openings that define lower vent passageways into a bottom of thecontainer cavity; and a second screen comprising a plurality of openingsthat define upper vent passageways into a top of the container cavity.14. The system according to claim 13 wherein a top portion of theextruded tubular wall comprises locking apertures, and the top capcomprises locking elements that are alterable between a retracted stateand an extended state, the locking elements biased into the extendedstate, and wherein the top cap is detachably coupled to the extrudedtubular wall when the locking elements are in the extended state andprotrude into the locking apertures.
 15. The system according to claim14 wherein contact between the extruded tubular wall and the lockingelements forces the locking elements into the retracted state duringinsertion of the top cap into the container cavity until the lockingelements become aligned with the locking apertures.
 16. The systemaccording to claim 14 wherein the bottom cap comprises the first screenand the top cap comprises the second screen.
 17. The system according toclaim 16 wherein the first screen is integrally formed into a body ofthe bottom cap and the second screen is integrally formed into a body ofthe top cap.
 18. The system according to claim 13 wherein the bottom capcomprises a floor plate that forms a floor of the container cavity andan oblique wall plate extending upward from a perimeter of the floorplate, the oblique wall plate coupled to the bottom end of the extrudedtubular wall, the first screen located on the oblique wall plate of thebottom cap.
 19. The system according to claim 13 further comprising afloor of the container cavity, and wherein at least one of the pluralityof openings of the first screen is located an axial distance above thefloor of the container cavity.
 20. The system according to claim 15,wherein the locking elements are spring biased latearlly outward towardsthe extended state.